The suggested proportion of vinegar (1/3 cup/gal) in the dye bath is about 1:50. Based on some unintentional experiments, it seems that this proportion is not that critical between, say, 1:20 and 1:100. But the result is much different if you forget the vinegar entirely.

Q: what does the vinegar do? Adjust pH? Mordant? Am I right to interpret the instructions as a proportion of vinegar to water in the dye bath (as opposed to proportional to quantity of dye)? I've been using generic white vinegar; anything better?

Hi,The vinegar changes the PH of the dye bath to an acid environment. Some dyes work better on certain fibers in an acid environment some work better in an alkaline environment. The proportion would be vinegar to water.Celia